The present invention relates to an image forming apparatus using an electrophotographic technology and a developing agent used in the image forming apparatus.
In a full color electrophotographic process, toners of four colors of cyan (C), magenta (M), yellow (Y) and black (K) are developed and fixed to obtain development of colors including intermediate color tones. In order to obtain a color tone having a high chroma, it is necessary for each toner to have a high transparency. It is also necessary to obtain a uniform fixed surface low in roughness.
In general, polyester resin or the like having a low molecular weight, which is likely to be subjected to a sharp melting, is used as a binder of toner for a full color electrophotography. However, the polyester resin leaves room for further improvement. Specifically, the polyester resin is low in its mechanical strength, tends to generate an offset problem, and is poor in compatibility with wax such as polypropylene (PP) having a low molecular weight or polyethylene (PE) having a low molecular weight. Also, the toner itself for the full color electrophotography is defective in that, since the pigment tends to be charged strongly, it is difficult to obtain a stable image density (ID).
As a measure for overcoming the offset problem in the case of using a colored toner, the thermal fixing roller is coated in general with a large amount of a silicone oil.
FIG. 1 exemplifies a fixing apparatus used in the conventional full color image forming apparatus. As shown in the drawing, the apparatus comprises a heat roller 35, a cleaning roller 4 arranged in contact with the heat roller 35, a means for supplying a silicone oil onto the heat roller 34, and a pressurizing roller 45 arranged to face the heat roller 35 and rotatable in synchronism with rotation of the heat roller 35. The silicone oil supply means comprises an oil coating roller 101 rotatable in synchronism with rotation of the heat roller 35, an oil impregnation tank 8 equipped with an oil impregnation roller 9 that is rotatable in synchronism with rotation of the oil coating roller 101, an oil transfer pipe 7 equipped with an oil transfer pump 6 and connected to the oil impregnation tank 8, an oil supply tank 5 connected to the oil transfer pipe 7 and housing a silicone oil, an oil recovery filter 102 arranged below the pressurizing roller 45, and another oil transfer pipe 103 connected at one end to the oil recovery filter 102 and at the other end to the oil supply tank 5.
In the fixing apparatus of the construction described above, the heat roller 35 can be coated with a large amount of the silicone oil supplied from the oil supply tank 5 via the oil coating roller 101.
However, a complex oil coating mechanism is required in the fixing apparatus shown in FIG. 1, making it necessary to conduct a maintenance operation such as replenishing of oil periodically. What should be noted is that, if the heat roller is abraded by the stress of the oil coating mechanism, the oil is attached to the printed sheet.
Such being the situation, it is proposed in recent years to add a rice wax that permits fixation without using an oil, said fixation being satisfactory in offset resistance, while maintaining a high transparency. However, the toner life is deteriorated in the case of adding a rice wax. Also, in the case of a full color toner, a soft resin is used. As a result, a so-called "spent toner", i.e., the phenomenon that the toner is adhered with time to the carrier and the developing apparatus, tends to take place. If a wax is added, it is made more difficult to prevent a charging capability of the full color toner from being lowered with time.
It is conceivable to add a charge controller, a hydrophobic silica, etc. in order to improve the toner life and fluidity of the toner. However, the charge controller is colored in many cases, and a colorless charge controller is incapable of producing a sufficient effect. Also, if a hydrophobic silica is used in a large amount, the charging amount is increased so as to lower the image density (ID). Further, the charging amount of the hydrophobic silica under a low humidity greatly differs from that under a high humidity, giving rise to additional problems such as fogging and scattering. Still further, even if the hydrophobic silica is used together with an inorganic oxide having a low resistivity such as titanium oxide, the surface resistivity of the toner is lowered. As a result, the charging amount tends to be lowered with time, and the toner scattering tends to take place.
As described above, it is difficult to maintain a sufficient offset resistance, to control the charging of a colored toner, and to prolong the toner life by preventing the occurrence of a spent toner.